The purpose of the present study is to analyze the mechanisms regulating fundamental properties of motor nerve terminals and muscle membrane as these mechanisms recover during the reinnervation process following a crush of the muscle nerves. These studies will be performed upon the extensor digitorum longus muscle and the soleus muscle of the rat which are fast (twitch) and slow (tonic) contracting muscles, respectively. The objectives of this project are: 1) to quantitatively evaluate an electrogenic pump which contributes to the resting membrane potential of single muscle fibers; 2) to determine the importance of the activity of such a pump to the overall physiological state of fast and slow muscles; 3) to compare the recovery during reinnervation, of the spontaneous release of acetylcholine with the release caused by a nerve action potential in order to separate the processes controlling these two types of transmitter release; 4) to examine the recovery of those mechanisms involved in the synthesis and release of acetylcholine from the regenerating nerve terminals in fast and slow muscles; 5) to determine the importance of transmitter release to the recovery of fast and slow muscle types during the reinnervation of these muscles following crush of their nerves.